Preparation of N-acylamino acid esters and N-acylamino acetals

ABSTRACT

A process for preparing N-acyl derivatives of the formula I,                    
     in which the substituents independently of one another have the following meanings: 
     X is CH(OR 3 ) 2 , COOR 3 ; 
     R 1  is hydrogen, C 1 -C 12 -alkyl, aryl, unsubstituted or substituted; 
     R 2  is hydrogen, C 1 -C 12 -alkyl, aryl, unsubstituted or substituted; 
     R 3  is C 1 -C 12 -alkyl, 
     which comprises reacting a carboxamide R 1 —CONH 2  of the formula II with a glyoxal monoacetal derivative of the formula III,                    
     in the presence of a carboxylic acid R 4 —COOH of the formula IV where R 4 =C 1 -C 12 -alkyl, where the substituents R 1  to R 3  are as defined above, is described.

The invention relates to a process for preparing N-acylamino acid esters and N-acylamino acetals.

A large number of different methods for synthesizing amino acids and their esters are known. A review is given, inter alia, in Ullmanns Encyclopedia of Industrial Chemistry, Vol. A2, 57-97, VCH Weinheim 1985.

Industrial syntheses of D,L-α-amino acids, for example the Strecker synthesis, use aldehydes as starting materials, which are reacted with NH₃ and HCN to give aminonitriles. The nitrile group can subsequently be reacted with alcohols or water to give the corresponding esters and amino acids, respectively.

DE-A-3145736 describes a process for preparing N-formyl-α-amino acid esters by reacting aminonitriles—for example from the Strecker synthesis—with an appropriate alcohol and formamide in the presence of hydrogen chloride.

Also known is the preparation of N-formyl-D,L-alanine from pyruvic acid by boiling with ammonium formate in formic acid [F. Yoneda and K. Kuroda, J. Chem. Soc. Chem. Commun., 1982, 927-929].

N-Formylalanine esters are used, inter alia, for preparing vitamin B₆ (Pyridoxine) [Review by König and Böll, Chem. Ztg. 100 (1976), 107/8] and isocyanic acid, for example according to Ugi, Angew. Chem. 77 (1965), 492.

The processes described have the disadvantage that the starting materials used are finished amino acids or precursors thereof—for example cyanohydrins or aminonitriles from the Strecker synthesis—which have to be prepared beforehand in a separate process.

It is an object of the present invention to provide a process for preparing N-acylamino acid esters and N-acylamino acetals which can easily be carried out on an industrial scale, using readily-obtainable starting materials.

We have found that this object is achieved by a process for preparing N-acyl derivatives of the formula I

in which the substituents independently of one another have the following meanings:

X is CH(OR³)₂, COOR³;

R¹ is hydrogen, C₁-C₁₂-alkyl, aryl, unsubstituted or substituted;

R² is hydrogen, C₁-C₁₂-alkyl, aryl, unsubstituted or substituted;

R³ is C₁-C₁₂-alkyl,

which comprises reacting a carboxamide R¹—CONH₂ of the formula II with a glyoxal monoacetal derivative of the formula III,

 in the presence of a carboxylic acid R⁴—COOH of the formula IV where R⁴=C₁-C₁₂-alkyl, where the substituents R¹ to R³ are as defined above.

Alkyl radicals for R¹ to R⁴ which may be mentioned are branched or straight-chain C₁-C₁₂-alkyl chains, for example methyl, ethyl, n-propyl, 1-methylethyl, n-butyl, 1-methylpropyl, 2 methylpropyl, 1,1-dimethylethyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl, 1-ethyl-2-methylpropyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl, n-undecyl and n-dodecyl.

The alkyl chains mentioned above can be unsubstituted, hydroxylated or substituted by mercapto groups. Preferred examples which may be mentioned are hydroxymethyl, hydroxyethyl, such as [CH₃—CH(OH)— or CH₂(OH)—CH₂] or mercaptomethyl radicals.

If the radical X in the formula I is CH(OR³)₂, the substituents R³ together with the oxygen atoms to which they are attached may also form a 5- or 6-membered ring. Starting materials used in this case are, for example, cyclic glyoxal monoacetals of the general formulae IIIa to IIIc.

Aryl for R¹ and R² is to be understood as an aromatic ring or ring system having 6 to 18 carbon atoms in the ring system, for example phenyl or naphthyl, which may be unsubstituted or substituted by one or more radicals, such as halogen, for example fluorine, chlorine or bromine, cyano, nitro, amino, C₁-C₄-alkylamino, C₁-C₄-dialkylamino, hydroxyl, C₁-C₄-alkyl, C₁-C₄-alkoxy or other radicals.

Preferred radicals for R¹ are hydrogen and the branched or straight-chain C₁-C₈-alkyl chains mentioned in the list above, particularly preferably C₁-C₃-alkyl chains. Very particularly preferred radicals for R¹ are hydrogen, methyl and ethyl.

Preferred radicals for R² are phenyl and the branched or straight-chain C₁-C₈-alkyl chains from the list mentioned above, particularly preferably C₁-C₃-alkyl chains. A very particularly preferred radical for R² is methyl.

Preferred alkyl radicals for R³ are the branched or straight-chain C₁-C₈-alkyl chains from the list mentioned above, particularly preferably C₃-C₈-alkyl chains, such as, for example, n-propyl, 1-methylethyl, n-butyl, 1-methylpropyl, 2-methylpropyl, 1,1-dimethylethyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, n-heptyl, n-octyl or 2-ethylhexyl.

Preferred radicals for R⁴ are the branched or straigh-chain C₁-C₈-alkyl chains from the list mentioned above, particularly preferably C₁-C₃-alkyl chains. Very particularly preferred radicals for R¹ are methyl, ethyl, n-propyl and isopropyl.

Depending on the amount of carboxamide R¹—CONH₂ and carboxylic acid R⁴—COOH employed, the formation of the different N-acyl derivatives of the formula I can be controlled in a targeted manner.

Thus, contrary to expectation, it has been found that reaction of an amount of carboxamide R¹—CONH₂ and carboxylic acid R⁴—COOH employed of in each case from 250 to 800 mol %, preferably from 400 to 600 mol %, based on the acetal of the formula II employed, gives N-acylamino acid esters of the formula I where X=COOR³.

A particularly advantageous embodiment of the process was found to be the use of the carboxamide R¹—CONH₂ and the carboxylic acid R⁴—COOH in identical molar proportions.

The process according to the invention is particularly suitable for preparing N-formyl-α-aminopropionic acid esters of the formula Ia

in which the substituent R³ is C₁-C₈-alkyl, preferably C₃-C₈-alkyl.

Formation of the N-acylamino acetals of the formula I where X=CH(OR³)₂ is preferred when the amount of carboxamide R¹—CONH₂ and carboxylic acid R⁴—COOH employed is in each case from 50 to 250 mol %, preferably from 100 to 200 mol %, based on the acetal of the formula II employed. In this case, too, it is particularly advantageous to employ carboxamide R¹—CONH2 and carboxylic acid R⁴—COOH in the reaction in a molar ratio of 1:1.

In the case of the N-acylamino acetals of the formula I, the process according to the invention is advantageously suitable for preparing N-formyl-2-aminopropionaldehyde acetals of the formula Ib

in which the substituent R³ is C₁-C₈-alkyl, preferably C₃-C₈-alkyl.

The conversion both into the N-acylamino acid esters and into the N-acylamino acetals is carried out at from 40 to 200° C., preferably from 60 to 150° C.

According to the invention, the reaction is carried out in a pressure range of from 200 to 1000 mbar, preferably between 500 and 1000 mbar, particularly preferably at atmospheric pressure.

The reaction can be carried out with or without additional solvent. The reaction is preferably carried out without adding a solvent.

Moreover, the process according to the invention can be carried out advantageously as a “one-pot process”, giving both N-acylamino acid esters and the novel N-acylamino acetals in excellent yields.

The isolation of the desired end product is carried out in a manner known per se. In the case of liquid reaction products, the esters or acetals formed are usually purified by distillation.

The invention also provides N-acyl derivatives of the formula Ic,

in which the substituents independently of one another have the following meanings:

R¹ is hydrogen, C₁-C₁₂-alkyl, aryl, unsubstituted or substituted;

R² is hydrogen, C₁-C₁₂-alkyl, aryl, unsubstituted or substituted;

R³ is C₁-C₁₂-alkyl.

Preference is given to N-acyl derivatives of the formula Ic, in which the substituents independently of one another have the following meanings:

R¹ is hydrogen, C₁-C₈-alkyl;

R² and R³ are C₁-C₈-alkyl.

With respect to the exact definition of the substituents R¹ to R³, both in the general and the preferred embodiments, the definitions given at the outset for the compound I should be referred to.

The N-acylamino acetals of the formula Ic are suitable for use as intermediates for preparing oxazoles.

The following examples are used to illustrate the subject matter of the present invention in more detail.

EXAMPLE 1 Butyl N-formyl-D,L-alaninate from Methylglyoxal di-n-butyl Acetal.

100 g of methylglyoxal dibutyl acetal (purity 93.5%, prepared according to EP 036539) were mixed with 100 g of formamide and admixed with 100 g of formic acid over a period of 10 min. The temperature of the mixture increased to 40° C., and the mixture was then heated to reflux temperature within 20 min. After a reaction time of 2 hours, the reaction mixture, which had been cooled to room temperature, was washed with dilute sodium carbonate solution, and the desired product was distilled under reduced pressure at 2 mbar. This gave 74.5 g of pure butyl N-formyl-D,L-alaninate (93% of theory).

EXAMPLE 2 2-ethylhexyl N-formyl-D,L-alaninate from Methylglyoxal di-2-ethylhexyl Acetal

50 g of methylglyoxal di-2-ethylhexyl acetal (purity 92%) were boiled under reflux with 30 g of formamide and 30 g of formic acid for 2.5 hours. The mixture was washed with 200 ml of sodium carbonate solution and distilled. From the main fraction, 29.8 g of 2-ethylhexyl N-formyl-D,L-alaninate (89% of theory) were isolated.

EXAMPLE 3 N-formylaminopropionaldehyde di-n-butyl Acetal from Methylglyoxal di-n-butyl Acetal

100 g of methylglyoxal dibutyl acetal (purity 93.5%, prepared according to EP 036539) were mixed with 50 g of formamide and admixed with 50 g of formic acid over a period of 10 min. The temperature of the mixture increased to 40° C., and the mixture was then heated to reflux temperature within 20 min. After a reaction time of 2 hours, the reaction mixture, which had been cooled to room temperature, was washed with dilute sodium carbonate solution, and the desired product was distilled under reduced pressure at 2 mbar. This gave 39 g of N-formylaminopropionaldehyde di-n-butyl acetal. 

We claim:
 1. A process for preparing N-acyl derivatives of the formula I,

in which the substituents independently of one another have the following meanings: X is CH(OR³)₂, COOR³; R¹ is hydrogen, optionally substituted C₁-C₁₂-alkyl, optionally substituted aryl; R² is hydrogen, optionally substituted C₁-C₁₂-alkyl, optionally substituted aryl; R³ is optionally substituted C₁-C₁₂-alkyl, or, when X denotes CH(OR³)₂, the substituents R³ together with the oxygen atoms to which they are bonded and with the carbon atom to which the oxygen atoms are bonded form a 5- or 6-membered ring; which comprises reacting a carboxamide R¹—CONH₂ of the formula II with a glyoxal monoacetal derivative of the formula III,

in the presence of a carboxylic acid R⁴—COOH of the formula IV where R⁴ is hydrogen or optionally substituted C₁-C₁₂-alkyl, where the substituents R¹ to R³ are as defined above.
 2. A process as claimed in claim 1, wherein the substituents have the following meanings: R¹ is hydrogen, optionally substituted C₁-C₈-alkyl; R² is optionally substituted C₁-C₈-alkyl, optionally substituted aryl; R³ and R⁴ are optionally substituted C₁-C₈-alkyl.
 3. A process as claimed in claim 2, wherein the substituents have the following meanings: R¹ is hydrogen; R² to R⁴ are C₁-C₈-alkyl.
 4. A process as claimed in claim 2 for preparing N-formyl-α-aminopropionic acid esters of formula Ia

in which the substituent R³ is C₁-C₈-alkyl.
 5. A process as claimed in claim 1, wherein the substituents have the following meanings: X is CH(OR³)₂; R¹ is hydrogen, optionally substituted C₁-C₈-alkyl; R² is optionally substituted C₁-C₈-alkyl, optionally substituted aryl; R³ and R⁴ are optionally substituted C₁-C₈-alkyl.
 6. A process as claimed in claim 5, wherein the substituents have the following meanings: R¹ is hydrogen; R² to R⁴ are C₁-C₈-alkyl.
 7. A process as claimed in claim 5 for preparing N-formyl-2-aminopropionaldehyde derivatives of the formula Ib

in which the substituent R³ is C₁-C₈-alkyl.
 8. A process as claimed in claim 2, wherein the amount of the respective carboxamide R¹—CONH₂ and carboxylic acid R⁴—COOH employed is from 250 to 800 mol %, based on the acetal of the formula II employed.
 9. A process as claimed in claim 8, wherein the amount of the respective carboxamide R¹—CONH₂ and carboxylic acid R⁴—COOH employed is from 400 to 600 mol %, based on the acetal of the formula II employed.
 10. A process as claimed in claim 8, wherein the carboxamide R¹—CONH₂ and the carboxylic acid R⁴—COOH are employed in the reaction in a molar ratio of 1:1.
 11. A process as claimed in claim 5, wherein the amount of the respective carboxamide R¹—CONH₂ and carboxylic acid R⁴—COOH employed is from 50 to 250 mol %, based on the acetal of the formula II employed.
 12. A process as claimed in claim 11, wherein the amount of the respective carboxamide R¹—CONH₂ and carboxylic acid R⁴—COOH employed is from 100 to 200 mol %, based on the acetal of the formula II employed.
 13. A process as claimed in claim 11, wherein the carboxamide R¹—CONH₂ and the carboxylic acid R⁴—COOH are employed in the reaction in a molar ratio of 1:1. 